The invention relates to a hydro-pneumatic front axle suspension system for the driven axle of an agricultural utility vehicle, in particular for the front axle of an agricultural tractor, with a pair of hydraulic cylinders coupled between vehicle chassis and axle body. The cylinders have a rod end chamber and a piston end chamber, each connected to at least one accumulator. A valve circuit supplied by a hydraulic pump controls the position of the chassis relative to the axle, and controls the pressure in the accumulators as a function of the load on the axle body.
In order to increase vehicle operational safety and comfort at higher vehicle speeds on roads, dirt roads and around curves, the front axle of agricultural utility vehicles can be resiliently or "spring" suspended from the vehicle chassis. For this purpose one or more hydraulic cylinders can be coupled between the vehicle chassis and the front axle, with piston end chambers and/or rod end chambers connected to accumulators and a position control valve system supplied by a hydraulic pump. For front loader operations or for operation in the ground the resilient suspension system is blocked mechanically or hydraulically (Peter Pickel et al. "What Chances do Spring-mounted Tractors Have" in the German periodical "Landtechnik" 10/90, pages 363-366). This is accomplished by blocking the supply lines to the hydraulic cylinder or by draining its piston end chamber (see DE-A-195 41 823), so that the chassis is firmly coupled to the front axle (see German periodical "Profi" Number 8/96, pages 14-17).
Such blocking of the spring suspension is necessary in order to avoid variations on the control performance of a power lift or hitch (see DE-A-38 34 693). For example, in DE-C-42 42 448, front axle spring suspension considered to be undesirable during plowing, and a manually actuated disengagement of the spring suspension is proposed.
In operations with front loader, front cultivator or front stacker a spring suspension is also considered to be detrimental since a particularly precise implement control and guidance is required in such situations. Hence, Prospectus of Fendt Company:"Farmer", HD11/95/15, on page 14, proposes to block the spring suspension with push-button actuation.
DE-A-43 08 460 proposes an operating device which enabled the operator to select between a manual and an automatic blocking of the spring suspension, where the automatic blocking occurs as a function of certain criteria, such as front loader operation or plowing.
Furthermore, DE-A-42 42 448 proposes, in case of electrical failure, to close a blocking valve between the hydraulic cylinder and accumulator automatically, so that the tractor remains ready to operate, although with reduced comfort.
EP-A-0 670 230 discloses a pressurized fluid supply for a hydro-pneumatic axle spring suspension with level control for a utility vehicle. This includes a hydraulic installation supplied by a hydraulic pump connected with accumulators that are in turn connected with chambers situated above and below the piston of actuating cylinders of the axle spring suspension. When the pressure falls below that required for the desired level, the upper cylinder chamber is filled by the hydraulic pump via the hydraulic installation. In order to maintain the supply of pressurized fluid, upon selective application of various pump systems, the actuation of a consumer connected to the hydraulic pump is arbitrarily simulated, in case of a need for pressurized fluid by the axle suspension. For certain operating applications the spring suspension is disengaged and the vehicle axle is blocked with respect to the vehicle chassis, by releasing the pressure in the upper cylinder chamber so that the pressure in the second cylinder chamber forces the axle against a stop on the chassis.
In front axle spring suspensions which are subject to heavy loads, such as are encountered in front loader operations or with the employment of a heavy front implement, in which the spring suspension is always blocked, the hydraulic and mechanical spring suspension components are designed for relatively low loads for reasons of space and cost, as would be required by operation on a road. In such a system the accumulators will have a relatively small volume with a correspondingly steep spring characteristic.